1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for heat spreading on the back surface of seeking missile window domes and radomes.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
During the flight of infrared (IR) and radio frequency (RF) seeking missiles for various engagement scenarios, the temperature of both the infrared window domes and RF ceramic radomes experience very high heat loads from the compressed air in flight, resulting in very significant temperature gradients across and through this section of the missile. These temperature effects are very transient during flight and also create significant stresses which can easily lead to structural failure of the missile. Much work has been reported on the behavior of these effects as reported by various US companies [C. A. Klein, “Infrared Missile Domes: Heat Flux and Thermal Shock”, SPIE Proceedings, Vol. 1739, pp. 230-253 (1992); R. L. Gentilman, et. al, “Thermal Shock Resistance of Convectively Heated Infrared Windows and Domes”, SPIE Proceeding, Vol. 3060, pp. 115-129 (1997); Ceradyne, Inc., “Radomes and Ceramic Radomes for Missile Systems”] and now the Chinese academic institutions. [J. Zhenhai, et al., “Thermal-structure analysis of supersonic dome based on three materials”, IEEE Article No. 5777830, 2011; W. Ziming, “The Calculating Models of Cooling IR Window and Window Background Radiation”, Vol. 3375, pp. 195-202 (1998)] Temperatures of these windows and radomes can be near and often exceed 1000° C., levels which can affect both their survivability and performance. Previously, there have been ablative approaches to remove the thermal heat from the radomes as described in U.S. Pat. No. 4,949,920 (Schindel); U.S. Pat. No. 5,340,058 (Holl); and U.S. Pat. No. 5,457,471 (Epperson), but the present inventive approach using advanced oscillating heat pipe technology coupled with forced convection of the “working fluid” offers a significant improvement. In addition, the present invention is applicable to both IR seeking window domes and RF guided radome type nose cone shells.
Features of the present invention include the design, fabrication and integration of active cooling heat spreaders on the back surface of missile domes, radomes, or windows to enhance their performance. FIGS. 1-4 illustrate the application of using these advanced heat spreaders to cool the missile dome or radome from their high temperature experienced during flight. This active cooling approach uses oscillating heat pipes (OHP) that have demonstrated effective thermal conductivities of 10-20,000 W/m·K. In addition, this active cooling component can be fabricated from same materials as the dome or radome, thereby matching the coefficient of thermal expansion (CTE). Such a condition can significantly reduce the transient and/or near-equilibrium stress in the dome or radome produced by thermal gradients during missile's travel at speeds of Mach 3-6. Another attractive operating feature of these closed-loops OH P's is that they perform better as the thermal heat density increases. Finally, integrating a very small pump, <1 inch3 in volume, such as DARPA's recently developed piezo-electric driven devices into the OH P's closed loop, the heat removal can be enhanced by forced convection (FC) of OHP's “working fluid” to heat removal greater than kW/cm2 from the back surface of the missile dome or radome.
Operation of Oscillating Heat Pipes and Forced Convective OHP or FC-OHP
FIG. 5 shows the basic concept principle of single loop oscillating heat pipes (OHP) and lists their important features. This advanced heat spreader uses capillary heat pipe (HP) action in a closed loop channel that contains no “wicks” like open loop heat pipes. The large “effective” thermal conductivity is created as a result of the unidirectional flow (often aided by internal check-valves) and the evaporation-condensation action of “working fluid”, i.e., the latent heat of vaporization, a thermal process called “oscillating motion of the liquid plugs and vapor bubble”. The pressure created during “working fluid” vaporization creates the oscillatory motion, typically 20 kHz. [C. Wilson, et. al., “Visual Observation of Oscillating Heat Pipes Using Neutron Radiography”, J. of Thermophysics and Heat Transfer, Vol. 22, pp. 366-372 (2008)] This “Heat Pipe Action” can be promoted by various “working fluids” such as water, acetone and ammonia at room temperature (RT), nitrogen for cryogenic operation and with alkali and other metal vapors at temperatures toward 100° C. and higher. Akachi as disclosed in U.S. Pat. Nos. 4,921,04 and 5,219,020 pioneered this new device, which utilizes the pressure change in volume expansion and contraction during phase change to excite the oscillation motion of the liquid plugs and vapor bubbles. This OHP has at least four important features that do not exist in regular heat pipes:
(1) OHP is an “active” cooling device, in that it converts intensive heat from the high-power generating device into kinetic energy of the “working fluid” in support of the oscillating motion;
(2) Liquid flow does not interfere with the vapor flow in high heat removal because both phases flow in the same direction;
(3) The thermally-driven oscillating flow inside the capillary tube will effectively produce some “blank” surfaces that significantly enhance evaporating and condensing heat transfer; and
(4) The oscillating motion (≅20 kHz) in the capillary tube significantly enhances the forced convection in addition to the phase-change heat transfer. [S. P. Dad, et. al., “Thermally induced two-phase oscillating flow inside a capillary tube”, Internat. J. of Heat Mass Transfer, Vol. 53, p. 3905 (2010); C. Wilson, et al., “Visual Observation of Oscillating Heat Pipes Using Neutron Radiography,” Journal of Thermophysics and Heat Transfer, Vol. 22, No. 3, pp. 366-372 (2008)].
Large heat transfer, however, does not exist for a single loop OHP of FIG. 5 but does with the multiple loop design show in FIG. 6. Here, the OHP is still a single closed loop and the “working fluid” oscillates back and forth through the loop thereby transferring heat energy from the evaporator to the condenser with an effective high thermal conductivity. This oscillation of the “bubbles” and “plugs” and convective movement provides significant heat transfer. FIG. 7 illustrates a “picture” of the dynamics of a capillary OHP having different regions of “bubbles” and “plugs” which can be viewed. These oscillations have been measured to be 20 kHz (5) corresponding to 50 μsec durations which promotes near isothermal conditions for the material containing the grooves of these advanced heat spreaders.
Major Technical Advance in High Thermal Conductivites: K-Values >10,000 W/m*° K
FIG. 8 shows recent data on the performance of a newly developed oscillating heat pipe (OHP) by Prof. Ma's group at the University of Missouri. It shows the first demonstration of an oscillating heat pipe having a thermal conductivity greater than 10,000 W/m*° K a value 4-5 times sapphire. This OHP, FIGS. 9a, -b and 10 was fabricated and operated in the following manner:
Total thickness is 3 mm and consists of a 0.5 mm top and 0.5 mm bottom plate bonded onto a 2 mm middle plate section having 0.76 mm square, connecting grooves on both sides as shown in FIGS. 9 and 10. The “working fluid”, acetone or water, was introduced via the fill-port, FIG. 9b, and the Flat-Plate (FP)OHP structure was Cu. These rows of grooves on both top and bottom (not shown) of the center piece shown in FIG. 9a are connected through grooves at the ends. FIG. 10 provides a cross section of this FP-OHP of FIG. 9b. Heat was applied by a 1 square inch heating element on one side of the heat spreader's evaporator section and heat was removed from the spreader's condenser. Very good insulation surrounding the heat spreader's edge adiabatic section assured negligible heat removal from the edges. Two cooling blocks attached to the spreader's condenser area were cooled with 60° C. water. The oscillating heat pipe had a dimension of 13 cm×4 cm and 0.3 cm thick. The key to fast flow is due to narrow grooves, nominally 700 microns diameter. This OHP, FIGS. 9a and b were tested in the following procedure: Heat was applied by a 1 square inch heating element on one side of the heat spreader's evaporator section, and heat was removed from both sides of the spreader's condenser section with good insulation surrounding the adiabatic sections. The approach for making such a “heat spreader” operate at higher powers, kW/cm2, necessary for the removal of high heating power density from the missile windows and radomes is to use a mechanically-controlled, two-phase heat pipe as shown in FIG. 11 and described in U.S. Pat. No. 8,213,471 (Schlie). An IR or RF missile integrated with this novel heat spreader having effective thermal conductivity Keff≅10-20,000 W/m·K will significantly lower the missile dome or radome, respectively, operating temperature. In addition, there should also be a significant increase in thermal conductivity when the heat flux increases as FIG. 8 shows.
The improved Oscillating Heat Pipe heat exchanger system shown in FIG. 11 using the mechanically controlled, two phase oscillating motion of the working fluid of the heat pipe can achieve much higher effective thermal conductivity and resultant heat transfer values, greater than kW/cm2, a value never before conceived or demonstrated. Further enhanced performance, however, can be achieved via use of nanoparticles and nanofluids inside of the working fluid. [H. B. Ma, et al., “Nanofluid Effect on the Heat Transport Capability in an Oscillating Heat Pipe,” Applied Physics Letters, Vol. 88 (14), p. 1161 (2006)] to acquire the lower temperature behavior of the backside of the missile window or radome. This effect is discussed below and data illustrating this enhancement is shown in FIG. 12. In addition, this integrated window or radome-unique heat pipe thermal management system allows good CTE (coefficient of thermal expansion). This condition exists since the material used to make the heat pipe system can be made from basically any material including the missile material itself as like similar dome material used for OHP bonded together, an arrangement which could provide nearly perfect CTE over the entire operating temperature range from sub-cryogenic to greater than room temperatures. In addition, a 50% improvement of OHP can be obtained using dispersed nanofluids in the working fluid of the OHP as shown in FIG. 13. High heat transport capability of nanofluids produced by adding only a small amount of nanoparticles into the fluid has qualified nanofluids as a most promising candidate for achieving ultra-high-performance cooling. and significant increase in critical heat flux (CHF) In addition, the heat transport capability in the nanofluid OHP depends on the operating temperature. When the operating temperature increases, the heat, transport capability significantly increases. In addition, the temperature difference between the evaporator and condenser is almost constant as the input power increases.
Referring back again to FIG. 5b, the figure highlights some of the major features of oscillating heat pipes. Of these properties, four of these are very significant, namely: there no wicks in the capillary or grooves of the OHP unlike conventional heat pipes; the external thermal heat for convective flow of the “working fluid” of the OHP; the system is a closed loop system; and “near perfect” CTE matching on bonded surface between materials occur for all temperatures.
The very large thermal conductivity of the above described Flat Plat-Oscillating Heat Pipe (FP-OHP), FIG. 8, can be fabricated from any material including the most common dome material, namely ZnS, Al2O3 or MgF2 or radome ceramic, fused silica or silicon nitride material.
Again, FIG. 11a illustrates a simple application of the OHP described in FIGS. 5-7 in which the heat source is a hot plate and jet impingement cooling with a cooled fluid like water. FIG. 11b shows the integration with an internal pump to enhance the OHP heat transfer [U.S. Pat. No. 8,213,471 (Schlie); U.S. Provisional Application Ser. No. 61/512,730] which does the following: rapidly removes heat from the dome with thermal conductivity >10,000 W/m·K; spreads and transfers this thermal energy to the two cooling blocks on each end of FC-OHP; and has heat removed from cooling blocks into the coolants.
Such a configuration can remove large heat intensities, greater than kW/cm2. The type of oscillating heat pipe shown in FIG. 11, operating only by the thermal excitation causing a net convective movement of the “working fluid” “bubbles” and “plugs”, cannot remove heat power flux levels more than 0.3 kW/cm2. Due to the limitations existing in the conventional single phase flow, vapor chamber and oscillating heat pipe, a novel mechanically-controlled hybrid oscillating two-phase system, as shown in FIG. 11, is employed for this High-Speed Radome invention. This type of mechanical driven by using an internal pump causes the oscillating “working fluid” to convectively move uni-directionally through the closed loop structure and capable of providing heat removal fluxes of greater than kW/cm2.
When heat continuously increases in the thermal load, such as in a missile dome or radome, currently available cooling devices such as liquid cooling such as used in the jet impingement cooling approach cannot meet the requirement. This is attributed to the capillary limitation, boiling limitation, vapor flow effect, and thermal resistances occurring in the wicks significantly limit the heat transport capability. Therefore, in order to develop a highly efficient cooling system to remove the extra-high heat flux and significantly increase the effective thermal conductivity, the mechanically controlled hybrid heat pipe of the invention is proposed and discussed in detail below. Later the details of the use of the a spherically configured equivalent oscillating heat pipe having features like the FP-OHP of FIG. 8 and FIG. 9 will be discussed. Here the application of the OHP to serve as an Advanced Heat Spreader will be integrated with dome structure of the missile improves its performance for reliability, improved tracking and increased speed.
Additional Aspects of Advanced Thermal Management with OHP
To provide a greater appreciation of the merits of the OHP of the invention, a discussion of certain main concepts must be provided, namely for thin film evaporation, thermally excited oscillating motion, nanofluid, and nanostructure-modified wicks.
Thin Film Evaporation.
In the presence of a thin film, a majority of heat will be transferred through a very small region. [M. A. Hanlon et al., “Evaporation Heat Transfer in Sintered Porous Media,” ASME Journal of Heat Transfer, 125, pp. 644-653 (2003); S. Demsky, et al., “Thin film evaporation on a curved surface”, Microscale Thermophysical Engineering, 8, 285-299 (2004); H. B. Ma, et al., “Fluid Flow and Heat Transfer in the Evaporating Thin Film Region,” Microfluidics and Nanofluidics, Vol. 4, No. 3, pp. 237-243. (2008)] When evaporation occurs only at the liquid-vapor interface in the thin-film region, in which the resistance to the vapor flow is negligible, evaporating heat transfer can be significantly enhanced, resulting in much higher evaporating heat transfer coefficient than boiling heat transfer coefficient with enhanced surfaces. [J. R. Thome, Enhanced Boiling Heat Transfer, Hemisphere Publishing Corporation, (1990) New York; R. L. Webb, 1994, Principles of Enhanced Heat Transfer, John Wiley & Sons, Inc, New York; M. Kaviany, 1995, Principles of Heat Transfer in Porous Media, Springer, New York; Liter, S. G., and Kaviany, M., 2001, “Pool-boiling CHF Enhancement by Modulated Porous-Layer Coating: Theory and Experiment,” International Journal of Heat and Mass Transfer, 44, pp. 4287-4311] Utilizing this information, a number of high heat flux heat pipes have been developed at the University of Missouri (MU). The micro-grooved heat pipe, 6-mm diameter and 135-mm length, for example, produces a temperature drop of only 2° C. from the evaporator to the condenser under a heat input of 50 W. The air-cooled aluminum heat pipe developed at MU, as another example, can remove a total power of 200 W with a heat flux up to 2 MW/m2. Utilizing and optimizing thin film regions will significantly increase the heat transport capability and effectively increase the effective thermal conductibility of the vapor chamber.
High Heat Transport Capability of Nanofluids.
High heat transport capability of nanofluids produced by adding only a small amount of nanoparticles into the fluid has qualified nanofluids as a most promising candidate for achieving ultra-high-performance cooling. Argonne National Laboratory [Choi, S. U.S., 1995, “Enhancing Thermal Conductivity of Fluids with Nanoparticles,” Developments and Applications of Non-Newtonian Flows, Amer. Soc. of Mech. Eng., New York, FED—Vol. 231/MD-Vol. 66, pp. 99-105] has demonstrated that the dispersion of a tiny amount of nanoparticles in traditional fluids dramatically increases their thermal conductivities. Since 1995, outstanding discoveries and seminal achievements have been reported in the emerging field of nanofluids. The key features of nanofluids discovered so far include thermal conductivities far above those of traditional solid/liquid suspensions [J. A. Eastman, et al., “Anomalously Increased Effective Thermal Conductivities of Ethylene Glycol-Based Nano-Fluids Containing Copper Nano-Particles,” Applied Physics Letters, Vol. 78, p. 718 (2001)]; a nonlinear relationship between thermal conductivity and concentration [S. U. S. Choi, et al., “Anomalous Thermal Conductivity Enhancement in Nano-tube Suspensions,” Applied Physics Letters, 79, pp. 2252-2254 (2001)]; strongly temperature-dependent thermal conductivity [S. K. Das, et al., “Heat Transfer in Nanofluids—A Review,” Heat Transfer Engineering, Vol. 27(10), p. 3 (2006)]; and significant increase in critical heat flux (CHF). [Y. Xuan, et al., “Investigation on Convective Heat Transfer and Fluid Features of Nanofluids,” Journal of Heat Transfer, 125, pp. 151-155 (2003); I. C. Bang, et al., “Boiling heat transfer performance and phenomena of Al2O3-water nano-fluids from a plain surface in a pool,” International Journal of Heat and Mass Transfer, Vol. 48 (12), p. 2407 (2005)] These key features make nanofluids strong candidates for the next generation of coolants to improve the design and performance of thermal management systems. Most recently, Ma's group at MU [Y. Zhang, et al., “Nonequilibrium heat conduction in a nanofluid layer with periodic heat flux,”: International Journal of Heat and Mass Transfer, Vol. 51(19-20), p. 4862 (2008); H. B. Ma, et al., “An Experimental Investigation of Heat Transport Capability in a Nanofluid Oscillating Heat Pipe,” ASME Journal of Heat Transfer, Vol. 128, p. 1213 (2006); H. B. Ma, et al., “Nanofluid Effect on the Heat Transport Capability in an Oscillating Heat Pipe,” Applied Physics Letters, Vol. 88 (14), p. 1161 (2006)] charged the nanofluids into an oscillating heat pipe (OHP) and found that nanofluids significantly enhance the heat transport capability in the OHP. When the nanofluid (HPLC grade water containing 1.0 vol. % 5-50 nm of diamond nanoparticles) was charged to the OHP, the temperature difference between the evaporator and the condenser can be significantly reduced. For example, when the power input added on the evaporator is 100 W, the temperature difference can be reduced from 42° C. to 25° C. It appears that the nanofluid can significantly increase not only the effective thermal conductivity, but also the convection heat transfer and the thin film evaporation in the OHP. The heat transport capability in the nanofluid OHP depends on the operating temperature. When the operating temperature increases, the heat transport capability significantly increases. The temperature difference between the evaporator and condenser was almost constant as the input power increases, and the investigated OHP with charged nanofluids can reach 0.028° C./W at a power input of 336 W, which might set a record of thermal resistance in the similar cooling devices.
Theoretical Approach (Modeling and Optimizing Design)
Thin Film Evaporation.
To confirm the superior capabilities of nanofluids in high-heat removal, Demsky and Ma's model [S. Demsky, et al., “Thin film evaporation on a curved surface”, Microscale Thermophysical Engineering, 8, 285-299 (2004)] was extended to explore evaporating heat transfer through a thin nanofluid film, assuming a 0.2% volume fraction of Al2O3 added into water as the working fluid. The heat flux in the thin-film region now peaks at 11.6 MW/m2 for a superheat of 1.0° C., over 50 percent increase than that in regular fluids, indicating that the nanoparticles can indeed significantly increase evaporating heat transfer through the thin film region. Most excitingly, as the liquid phase continuously vaporizes and consequently the volume fraction of nanoparticles in the thin film region further increases, the effective thermal conductivity of nanofluids becomes higher which may result in even higher heat transfer rates than that [H. B. Ma, et al., “An Experimental Investigation of Heat Transport Capability in a Nanofluid Oscillating Heat Pipe,” ASME Journal of Heat Transfer, Vol. 128, p. 1213 (2006)] Effective heat removal also assures temperature uniformity across the evaporating section. Higher thermal conductivity of nanofluids, in addition, will reduce the thermocapillary flow in the thin film region, which significantly assists the nanofluid in passing the thin film region and thus remarkably raise the dryout limit. Using the newly developed model [I. C. Bang, et al., “Boiling heat transfer performance and phenomena of Al2O3-water nano-fluids from a plain surface in a pool,” International Journal of Heat and Mass Transfer, Vol. 48 (12), p. 2407 (2005)], heat transfer and fluid flow in thin film region occurring in the nanostructure wicks will be predicted and the optimum design for the wicks to be used in the evaporating section of the proposed system will be obtained.
Thermal Modeling of Oscillating Motion.
In order to exploit the superior performance of nanofluids for heat transfer enhancement, a number of nanofluid OHPs shown in FIG. 9 was developed and tested in Dr. Ma's Lab and found that the nanofluids significantly enhance the heat transport capability in an oscillating heat pipe [Borgmeyer, B. et al., “Experimental Investigation of Oscillating Motions in a Flat Plate Pulsating Heat Pipe,” AIAA Journal of Thermophysics and Heat Transfer, Vol. 21, No. 2, pp. 405-409, (2007); K. Park et al., “Nanofluid Effect on the Heat Transport Capability in a Well-Balanced Oscillating Heat Pipe,” AIAA Journal of Thermophysics and Heat Transfer, Vo. 21, No. 2, p. 443 (2007); Qu, W. et al., “Theoretical Analysis of Start-up of a Pulsating Heat Pipe,” International Journal of Heat and Mass Transfer Vol. 50, pp. 2309-2316 (2007)]. Experimental results show that the nanofluid can enhance the oscillating motion of working fluid in the OHP and the temperature difference between the evaporator and the condenser can be reduced significantly as shown in FIG. 12. Clearly, the combined strengths of high-conducting nanofluids, superior evaporating heat transfer rate in thin nanofluid films, and oscillations have resulted in an excellent cooling rate. Examining the total thermal resistance from the evaporator to the condenser, the thermal resistances for the wall were the same for both cases. In other words, the decrease of temperature difference was only from the fluid phase. This means the nanofluids have much more of an effect than the data shown in FIG. 13.
Features of the Inventive High Performance Oscillating Heat Pipe System for Lower Temperature Operation of Missile Window Domes or Ceramic Radomes
The mechanically-controlled two-phase oscillating motion of the invention can reach a very high flow rate, which can reach an extra high level of temperature uniformity resulting in higher than all other kinds of heat pipes including the standard vapor chamber.
The hybrid system of the invention utilizes both the sensible and latent heats to transport heat from the hot area to the cold area while the conventional heat pipes including the vapor chamber transport heat only by the latent heat. Due to the latent heat, the temperature distribution can reach a high level of uniformity. The preferred nanostructures modify the evaporating surface and maximize the thin film evaporation, resulting in an unprecedented evaporating heat transfer rate.
Due to the oscillating motion, the nanofluid can be used, which will significantly increase the heat transport capability. The plasma-nano-coated surface can modify the condensing surface resulting in high condensing heat transfer rate. Due to the two phase system, the pressure drop is much lower than that of single liquid phase, which can produce an extra high flow rate.
The system of the invention effectively integrates extra high level of heat transfer rate of thin film evaporation, high thermal transport capability of nanofluids, low pressure drop, and strong oscillating motions controlled by mechanical system, which can result in an extra high heat transport capability.
In addition to the phase change heat transfer, the strong oscillating motion of nanofluids existing in the system of the invention results in additional vortex in the liquid plugs significantly enhancing the heat transfer rate.
FIG. 11a illustrates a simple application of the OHP described in FIGS. 5-10 where the heat source is a hot plate and jet impingement cooling with a cooled fluid like water. FIG. 11b shows an application to a missile flat dome as an illustrative example. Under the dome window which may be at 1000° C., a bonded convectively forced OHP's “working fluid” evaporation-condensation process would do the following: rapidly removes heat from the dome with thermal conductivity >10,000 W/m·K; spreads and transfers this thermal energy to the two cooling blocks on each end of FC-OHP; and has heat removed from cooling blocks into the coolants.
Such a configuration can remove large heat intensities, greater than kW/cm2. For the use with this invention, the dome would either a truncated hemisphere or configured a conformal optic dome have more of pointed center shape. These current type of oscillating heat pipe operating only by the thermal excitation causing a net convective movement of the “working fluid” “bubbles” and “plugs” cannot remove heat power flux levels more than 0.3 kW/cm2. Due to the limitations existing in the conventional single phase flow, vapor chamber and oscillating heat pipe, a novel mechanically-controlled hybrid oscillating two-phase system, as shown in FIG. 11, are preferably employed for the invention. This type of mechanical driven by using an internal pump causes the oscillating “working fluid” to convectively move uni-directionally through the closed loop structure and capable of providing heat removal fluxes of greater than kW/cm2.
For the internal pump, a very small piezoelectric actuator pump operating at high pressure and speed is preferred. Such small pumps, such as shown in FIG. 15, create pressures greater than 2500 psi and are approximately 0.6 inch diameter and 1 inch in length plus have an inlet and outlet port which is compatible with the above described OHP. These pumps are available from, for example, Kinetic Ceramics, Inc in Hayward, Calif. These types of piezoelectric fluid pumps have the following unique features:
Solid-state piezoelectric drive with direct electromechanical energy conversions
No electromagnetic fields
High power density and high efficiency
Fast starting times and no electric motor or solenoids
Robust and reliable operation
High output pressure and flow rates
Output pressure to 2500 psi
Flow rates to 40 cc/sec
Small dimensions and aluminum housing
Weights of 275-450 grams
Compact electronic drives with less space required
Metering capability of pressure and flow velocities
Concept for Integrating Oscillating Heat Pipe with Dome Window
To appreciate the value of these types of oscillating heat pipes with their thermal conductivities greater than 10,000 W ° K, an analysis of the temperature profile and thermally induced strain or deformations effects was made. FIG. 1b depicts one side of an axisymmetric, cross-sectional view of a conceptual missile design having an oscillating heat pipe integrated with a spherical dome missile. The oscillating heat pipe is placed on the back surface of the dome which is connected to a cooling block bonded to the inside of the missile casing shown in FIG. 1b which is different from FIG. 1a having no such OHP. In this conceptual missile of length 1 meter and 8 cm diameter of FIG. 1b, the missile has an 80 mm diameter ZnS spherical dome window of 6 mm thickness. It is assumed that it is heated uniformly across the entire surface to 1000° C. (≅1250° K) by the near Mach 3-6 flow conditions. The aluminum side casing is 4 mm thick. On the back surface of the ZnS dome of 6 mm thickness is the 2 mm thick convectively forced OHP with K=10,000 W/m·K effective thermal conductivity. This configuration is similar in concept to the design shown in FIG. 11b for a mechanically controlled, Forced Convective Oscillating Heat Pipe, namely FC-OHP. Very important to note is that the OHP bonded inside the hemispherical dome covers all of the dome inside surface and hemispheric. The internal pump, like illustrated in FIG. 15, is not shown but identified by the arrowed block called “Internal Pump in OHP or Cooling Block”. The thermo-mechanical values for aluminum were Poisson ratio=0.33, Young modulus=70.3 GPa and coefficient of thermal expansion (CTE)=23.6 μm/m·K while for ZnS, these respective values were, 0.27, 114.37 GPa and 10.4 μm/m·K. Only axisymmetric conditions are considered in this cw analysis. The cooling blocks are to be cooled by aerodynamic cooling effects on the outside aluminum casing which can be utilized surface radiators on external on internal part of missile casing walls or sides.
Analysis of Thermal Behavior for Integrated OHP with Missile Dome Window
For comparison purposes, the analysis was performed for both case of FIG. 1 for conditions without and with the OHP bonded on the inside surface of the spherical of the ZnS dome window. Also for simplicity, it was assumed that cooling block still existed on the aluminum casing walls (FIG. 1b) and for the case of no OHP, the 2 mm thickness was replaced by ZnS thus making it also 8 mm thick. The result without the OHP is that the entire dome becomes in equilibrium the same temperature as the front surface, i.e., 1000° C., FIG. 16. In addition, at the junction at interface between the ZnS dome and the aluminum casing has significant temperature gradient which results in strong thermally induced stresses. Similar analysis for ZnSe and CaF2 produced similar behavior. For brevity, only the ZnS results are presented here. Using the OHP with an “effective” K=10,000 W/m·K illustrates the real value of this thermal management approach as show in FIG. 17 showing the significantly lower temperature of dome. With the cooling blocks at 100° C., enable by aerodynamic cooling of the outside air, the entire ZnS back surface is reduced down to by at least by 50%, a truly very valuable feature. Benefits of this reduced temperature is reduced large thermal stresses plus less blackbody radiant heat introduced into the infrared tracking by the radiating ZnS back surface into the sensor system of the missile. FIGS. 16-19 show the computed temperature profiles for various conditions, namely, FIG. 16 is case for no cooling, FIG. 17 is for a 2 mm active cooler-heat spreader bonded on back side of a 6 mm thick ZnS and with a K=10,000 W/m·K. FIG. 18 is same as FIG. 17 except K=20,000 W° K and FIG. 19 is for 4 mm active cooler-heat spreader bonded on back side of a 4 mm thick ZnS with K=20,000 W/m·K. It is easy to notice the benefits of the use of these oscillating heat pipes to remove heat from the dome which will also reduce the thermally induce stresses.
In FIG. 20, the deformations or displacements in the R-directions are shown for the two cases of (1) no active cooler-heat spreader, FIG. 20a which is related to temperature profile in FIG. 16 and (2) for a 4 mm active cooler-heat spreader bonded on back side of a 4 mm thick ZnS with K=20,000 W/m·K which is related to the temperature profile of FIG. 19. For case (1) a maximum R-displacement value of 780 microns is experienced in the ZnS dome windows but this is reduced to a maximum of 160 microns for case (2), a very significant difference which means less thermally created strain in the ZnS. Although there are different configurations of OHP with missile dome windows, the possible design tradeoffs offers the potential to establish an optimum design configuration to enhance the performance of actively cooled missile via the information provided in this invention. All of the results of FIGS. 1, 16-20 illustrate the value of integrating OHP with the window dome or radomes of the either heat seeking or RF guided missiles.
Based on the experimental investigation by Ma [Liter, S. G., and Kaviany, M., 2001, “Pool-boiling CHF Enhancement by Modulated Porous-Layer Coating: Theory and Experiment,” International Journal of Heat and Mass Transfer, 44, pp. 4287-4311 (2001)], the diamond nanoparticles are preferred because Ma and his researchers have conducted reliable tests for the nanofluid oscillating heat pipe and found the heat transfer performance is constant over the two-year testing. In other words, the evidence shows that the nanofluid (diamond nanoparticles) oscillating heat pipe has not deteriorated from the available tests. The diamond nanoparticles can be purchased from Nano Plasma Center Co., Ltd. with a very low price. While the diamond with a large size is very expensive, the price for diamond particles with a size about 50 nm is very low. For example, 500-gram diamond nanoparticles cost about $150, which can make over 100 heat spreaders proposed herein. Using the lathe, milling machine, and high temperature brazing furnace equipped in ThermAvant, the oscillating heat pipe similar to those shown in FIG. 18 can be readily fabricated. In order to develop a low cost fabrication process, the fabrication processes of casting and mold, milling by the lathe and milling machine, and brazing will be conducted.
The design of the OHP for the missile domes and radomes preferably utilizes the extra-high evaporating heat transfer of thin film evaporation, strong oscillating motion, higher heat transport capability of nanofluids, and nanostructure-modified surfaces and wicks to significantly increase the heat transport capability in the proposed hybrid phase-change heat transfer device. FIG. 27 illustrates six designs for the hybrid mechanically-controlled oscillating two-phase system schematically illustrated in FIG. 11. In order to form a very strong oscillating motion with high frequency, the oscillating motion will be mechanically controlled. A train of vapor bubbles and liquid plugs will flow through the channel with high speed. The channel wall, which will be fabricated from the microstructured wick, the solid line shown in six version for OHP shown in FIG. 27 will be used as the evaporating surface. The channel shape, channel arrangement, and channel number will depend on the total power, heat flux level, and heat sources. The open region between the liquid plugs will have an extra fast evaporation rate through nanostructure wicks. The pore size in the evaporating section must be optimized in order to have the maximum number of the thin film regions, excellent wetting characteristics, and optimum thickness for the maximum boiling limit. The wicks in the condensing area must be optimized to significantly increase the condensing heat transfer by using hydrophobic surfaces. The wetting characteristics will gradually vary from the perfectly wetting condition (hydrophilic) in the evaporating section to the partially wetting (hydrophobic) in the condensing section. The flow path for the liquid flow from the condensing section to the evaporating section will be optimized to significantly reduce the pressure drop.
When heat is added on the evaporating region of the microstructured surface from the heat source, as shown similar to FIG. 9 for the different version shown in FIG. 27, the heat is transferred to both the liquid plugs (sensible heat) and the region between the liquid plugs (latent heat). As the heat reaching the region between the liquid plugs, it is transferred through the nanostructure wicks and to the liquid-vapor interface, where the thin evaporation heat transfer occurs. The vapor generated in these areas will be immediately removed by the mechanically-controlled two-phase flow, and directly brings the heat from the evaporating (hot) area to the condensing (cold) area and condenses into liquid. The condensate is preferably pumped back by the mechanical controlled oscillator. When the total power and the heat flux level are high, the capillary force produced in the wick cannot overcome the pressure drop in the wick, the vapor chamber will reach the capillary limit, which is the reason why the current available heat pipe, although it is much better than single phase heat transfer cannot remove the heat at an extra-high level of heat flux such as occurring in the high power TDLs. When the input power is higher, while the oscillating motions of liquid plugs and vapor bubbles produced by the mechanically controlled oscillator can remove heat by the forced convection, it can directly help to bring condensate back to the evaporating surface. More importantly, the oscillating motion can make it possible to use nanofluid as the working fluid. Although the nanofluid has been introduced about 10 years ago, no application is available until the nanofluid oscillating heat pipe is developed. [K. Park, et al., “Nanofluid Effect on the Heat Transport Capability in a Well-Balanced Oscillating Heat Pipe,” AIAA Journal of Thermophysics and Heat Transfer, Vo. 21, No. 2, p. 443 (2007); W. Qu, et al., “Theoretical Analysis of Start-up of a Pulsating Heat Pipe,” International Journal of Heat and Mass Transfer Vol. 50, pp. 2309-2316 (2007; H. B. Ma, et al., “Heat Transport Capability in an Oscillating Heat Pipe,” ASME Journal of Heat Transfer, Vol. 130, No. 8, p 081501 (2008)). The reason is that when the nanoparticles settle down, the heat transport capability of nanofluid significantly reduces. For the proposed system, the oscillating motion excited by the oscillator (which can reach a high frequency) directly disturb the nanoparticles and make the nanoparticles to be suspended in the base fluid. The addition of nanoparticles into the base fluid can further increase the heat transport capability of the thin film evaporator on the microstructured surface. Due to the oscillating motions, the nanoparticles will not settle down. It should be noted that the nanoparticles will be added into the base fluid to be used in the proposed system.
The oscillating heat pipe charged with nanofluid preferably comprises three sections, i.e., evaporating section, adiabatic section, and condensing section. A cooling block connecting to a cooling bath is used to remove heat from the heat rejection section. Because the operating temperature directly increases the effective thermal conductivity of nanofluid and at the same time it will directly reduce the nanofluid viscosity, the operating temperature might have a significant effect on the heat transport capability. Using cooling blocks, the operating temperature are being varied from sub-zero to 200° C. Cryogenic operation also easily operate and often perform much better because most gaseous “impurities in the “working fluid” are frozen out. [H. Xu, et al., “Investigation on the Heat Transport Capability of a Cryogenic Oscillating Heat Pipe and Its Application in Achieving ultra-Fast Cooling Rate for Cell Verification Cryopreservation,” Cryobiology, Vol. 56, pp. 195-203 (2008)].
The experimental results show that the turn and length of heat pipes directly affect the heat transfer performance of OHPs. For the nanofluid OHP, the heat pipe turn ranging from 4 to 20 are experimentally investigated in order to find the optimum turn number. In order to test the heat pipe, an experimental setup shown in FIG. 18 is being constructed consisting of a test section, a data acquisition system, a cooling bath, and a power supply and measurement unit. A test section consisted of the heat spreader embedded with nanofluid OHP proposed herein, cooling blocks, a heater and supporter. The test section supporter will be machined from an aluminum block and designed to allow for various setups. The two cooling blocks can easily be moved to either increase or decrease the size of the condenser region. A heater can easily be removed and replaced by one of a different size. The heater simulating the computer chip will be placed directly in the center of the heat spreader with adequate contact between the heater and heat spreader embedded with nanofluid OHPs. The heater will be surrounded by insulation to insure the heat flux is directed into the heat spreader. A Julabo cooling bath equipped in the company will be connected to the two aluminum cooling blocks in order to maintain a constant condenser temperature. A variable power supply is preferably connected to the heater which will be also connected to a digital multi-meter. This will be to precisely determine the heat input to the heat spreader embedded with OHPs. Multiple type T thermocouples will be placed in the evaporator, condenser, and adiabatic regions to monitor the temperature throughout the heat spreader. An lotech data acquisition system will be used to obtain temperature readings from the thermocouples.
Required Properties of “Working Fluid” for Oscillating Heat Pipe
The selection of the correct working fluid is critical and preferred candidates have been identified. Its importance is due to the monitoring radiation must propagate through both the ZnS (and others like MgF2 or Al2O3) and the OHP containing the “working fluid”. Water and acetone are totally unacceptable due to their strong absorption. Some fluorocarbon liquids like FC-72 and FC-75 appear to be quite promising. For example, FC-75 H. Yoshida, et. al., “Heavy fluorocarbon liquids for a phase-conjugaged stimulated Brillouin scattering mirror”, Applied Optics, Vol. 36, p. 3740 (1997)] has been used as a nonlinear liquid inside high power laser and FC-72 has already been demonstrated to be good “working fluid” for oscillating heat pipes.
Minimizing Diffractive Effects Created by OHP Structure
Integrating the grooved structure of the OHP shown in FIGS. 9 and 27 with the dome window or radomes will create diffractive effects in the amplitude and phase of the detected infrared and RF radiation being detected. Fortunately, because the RF radiation is large relative to groove space, sub-millimeter relative to centimeter for RF wavelengths, the concern deals predominantly with the IR tracking missiles. Fortunately, the use of micro-lens diffuser provides an excellent way to overcome any such diffractive effects for IR seeking missiles. FIGS. 21-22 show the excellent aspects of this technology. In addition these micro-lens diffuser can homogenize laser radiation from many sources to produce amplitude 2-D beam shapes with less than 1% non-uniformity. [http://www.suss-microptics.com] Also, the resultant radiation is totally incoherent. [B. Kohler, et. al., “11 kW direct diode-laser system with homogenized 55×20 mm2 Top-Hat intensity distribution”, Proc. SPIE 6456, paper 6456-22, (Feb. 7, 2007); D. Shafer; “Gaussian to flat-top intensity distributing lens”; Optics & Laser Technology, Vol. 14, Issue 3, pp. 159-160, (June 1982); M. Traub, et al., “Homogenization of high power diode laser beams for pumping and direct applications”; Proc. SPIE Vol. 6104, (February 2006)].
These micro-lenses can be made from silicon which would be a good IR transmissive material and very promising for integrating with AHS-OHP for cooling IR missile domes.